The Absence of need, combined with strict control of the mentally ill.
In order to set aside "alien mentality" reasons, I have to assume their mentality is the same as modern humans, with all of our insanities, jealousies, greed and murderous impulses still intact, for every generation.
So the main idea here is to eliminate the reasons we humans go to war in the first place.
I think people go to war for two basic reasons. (1) They need something for survival (or continued life-as-they-know-it): Water, land, a food supply or other resources (like oil), or ... (2) They irrationally want wealth, resources they do not need, or desire the power to rule over and control the behavior of other people (by their own ideology or religion), as ends in themselves.
Imagine a world in which all energy is free and unlimited. The capturing of solar power, or some fusion engines. Add to this world sophisticated robotics that can do 100% of the work, and 100% of the distribution of products. Because the energy costs nothing, the robots cost nothing, the products cost nothing, the food costs nothing, the medicine and medical care all cost nothing. Technically, no "economy" is required. Nobody has to work a day in their lives. Nobody has to pay for anything. Nobody will go hungry, or without shelter, or without medical care or drugs they need or mental health care, or without all the education they desire, or without comfortable transportation anywhere on the planet they desire to go, or without entertainment, for that matter.
Although such a society could be more modern than ours in terms of electronics and entertainment, presume for this argument it has, along with highly advanced robotics (that will never try to kill them) all the most modern devices we have, Internet, mobile phones, air conditioning and heating, self-driving electric cars.
Some reasons may be left for violence (jealousy over a mate, for example), but what reasons are left for actual war?
The main thing I imagine is megalomania (the desire for power for its own sake), insatiable greed, murderous intolerance (often driven by race or religion), psychopathic leadership that enjoys causing death, and other "runaway" emotional states and irrational belief systems that I personally consider to be mental illnesses.
Those can be addressed in this society also, and such people can be restrained, by drugs, monitoring technology, prison or just put to death when they commit a crime. Some of these mental conditions have been traced to structural problems in the brain. We humans are not advanced enough to correct those structural defects, but future technology might be able to, or might identify such defects in the womb and abort the child.
They (like us) will always have amongst them psychopaths, sociopaths and other forms of insanity or compulsion (like rapists, or people that get a thrill from causing misery, terror, pain, and death). Because of that, I cannot imagine a peaceful society that abstains from all violence against its own people. No matter how good its defenses against off-planet invaders, they must still remain vigilant against the criminally insane within their own ranks, and many forms of this insanity can only be contained by violent apprehension (which could be by robots) and requires coercive restraint (prison) or execution.
Law enforcement does not constitute "war" in my view, a society can be peaceful and still have some bad apples willing to kill over jealousy or pride. But by identifying such mental illness, we can prevent such people from gaining power in the leadership and manipulating the society into war for irrational reasons.
It is, in my view, entirely possible to have a world of people that are content in what I'd call the modern middle class if they have no worries about their future safety or survival. Although nearly everybody entertains ideas of being rich or a celebrity or beloved by society, most never get that and are still satisfied with their life, their mate and family, and their place amongst the common folk that will, centuries later, be forgotten.
I include myself in this category; while I have invented, educated, entertained and mated and had some commercial successes, I suffer no delusions of gaining immortal fame (and see no real value in doing so). Yet I am content to continue in my current role and vein for life, without ever achieving it: I like to solve puzzles, be creative and entertain people, pursue ideas I think can help others solve problems. So I think if I can come to grips with my own mortality and eventual obscurity, so can everybody else that is not mentally ill, and a society such as the one I described above is certainly possible: A world of sufficiency and comfort that requires no labor to survive, that minimizes suffering, and prevents the mentally ill from gaining power in politics, business or private life. One that identifies and controls the criminally insane quickly and thoroughly, through the use of force when necessary.